For isolation of a structure in a non-volatile semiconductor memory (e.g., flash memory), a trench isolation is commonly employed. The trench isolation, however, has a problem that a resistance of a source diffusion layer is increased when the source diffusion layer is formed by implantation using self-align source etching that employs anisotropic dry etching. Moreover, with the trench isolation, the side wall of the trench is inclined sharply at an angle close to 0°. Therefore, it has been difficult to form a low-resistance source diffusion layer wiring on the side wall by ion-implantation.
As a countermeasure, Japanese Unexamined Patent Publication No. 2003-37193 (published on Feb. 7, 2003) teaches a method in which the isolation layer has a protrusion portion and a depressed portion, and the source diffusion layer is formed below the level of a bottom surface of the depressed portion. With this method, the resistance of the source diffusion layer can be reduced.
However, in order to form the source diffusion layer in such a deep portion, high energy is required for ion-implantation of impurity. The publication teaches a method in which the implantation is performed, using ion-implantation energy of 120K eV, from two directions at an angle of 45°. However, in the case where the diffusion layer is formed by ion-implantation, the diffusion layer expands in dependence upon the ion implantation energy, along a direction orthogonal to the direction of the ion-implantation. Therefore, if a source region is formed using a high energy such as 120K eV, the source region would expand along the direction orthogonal to the direction of extension of the source diffusion layer. This makes it difficult to shorten the width of the gate line by taking advantage of a short channel effect. Thus, the semiconductor memory cannot be miniaturized.
Further, Japanese Unexamined Patent Publication No. 2000-36546 (published on Feb. 2, 2000) teaches a structure in which a side wall of trench has a greater angle than the ion-implantation angle. With this structure, a source diffusion layer with a low resistance is formed. This publication teaches setting the ion-implantation angle at 7° so that (i) extrusion of ions can be prevented and (ii) efficiency of the implantation can be improved. However, as the implantation angle becomes wider, the number of ions implanted on the right trench side-wall layer and that implanted on the left trench side-wall layer become increasingly different. This increases the resistance in the side wall having a lower density. As a result, the resistance of the source diffusion layer is increased.